Process for making nanocone structures and using the structures to manufacture nanostructured glass
Fabrication method. At least first and second hardmasks are deposited on a substrate, the thickness and materials of the first and second hardmask selected to provided etch selectivity with respect to the substrate. A nanoscale pattern of photoresist is created on the first hardmask and the hardmask is etched through to create the nanoscale pattern on a second hardmask. The second hardmask is etched through to create the desired taper nanocone structures in the substrate. Reactive ion etching is preferred. A glass manufacturing process using a roller imprint module is also disclosed.
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This application claims priority to U.S. Provisional application Ser. No. 61/477,792 filed on Apr. 21, 2011 and to U.S. provisional application Ser. No. 61/491,939 filed on Jun. 1, 2011, the contents of both of which are incorporated herein in their entirety by reference.
This invention was made with government support under Grant No. W911NF-07-D-0004 awarded by the Army Research Office. The government has certain rights in this invention.BACKGROUND OF THE INVENTION
This invention relates to methods for making high aspect ratio nanocone structures that can be used to imprint nanoscale patterns on glass in an integrated glass manufacturing system.
Nanostructured surfaces have been widely studied for their superior optical and wetting properties such as antireflection and superhydrophobicity/hydrophilicity [1, 2, 3]. Due to their subwavelength feature size, such nanostructures behave as an effective medium with gradually varying index of refraction. Such a surface can be used to suppress Fresnel reflection at material interfaces, thereby acting as an antireflection surface and allowing broadband light to pass through without reflection losses . In addition, both hierarchical roughness from those structures and the intrinsic chemical property of the surfaces can induce artificial super hydrophobicity and/or superhydrophilicity which can be applied as self-cleaning and anti-fogging surfaces, respectively [5,6].
Although these notable properties of multi-functional surfaces are well understood, fabricating defect-free nanostructured surfaces with multiple functionalities remains a difficult engineering challenge as a result of limitations of existing nanofabrication methods. The performance of these nanostructured surfaces are determined by their geometry. It is thus necessary to fabricate gradually tapered structures with small feature sizes (Λ) and large height (H). The higher the aspect ratio (H/Λ) the structures have, the better the optical and wetting properties they exhibit .
Nanostructured surfaces can be prepared by several existing fabrication methods such as electron beam lithography , nanoimprint/polymer replication , deposition of multilayer porous films or chemical materials , and colloidal lithography . However, it is difficult to achieve high aspect ratio structures (greater than five) with a gradual tapered profile using prior art techniques. That is to say, the properties of subwavelength nanocone structures fabricated using existing techniques have limited performance.
Natural materials often have hierarchical structures on their surfaces. For example, a lotus leaf [5, 14] has hierarchical microstructures on its surface which keeps the plant clean for photosynthesis. These structures employ both material and geometric effects to render the surface superhydrophobic, thereby allowing water droplets to form spherical beads to remove surface particle contaminants. Using similar design principles it is possible to engineer a textured superhydrophobic surface that can self-clean. Such nanostructures can also be rendered superhydrophilic by controlling the surface treatment thereby allowing the surface to be anti-fogging.
By understanding these nature-inspired principles, it is possible to design surfaces that have combined wetting (self-cleaning and/or anti-fogging) and optical (antireflection, lossless transmission) properties. Using advanced lithography and multiple plasma etching processes, the methods disclosed herein are able to produce glass that is anti-glare, near-perfect transmitting, and selectively self-cleaning and/or anti-fogging.
It is therefore an object of the present invention to disclose methods for making high aspect ratio nanocone structures and using the structures to imprint a pattern during glass manufacture.SUMMARY OF THE INVENTION
In a first aspect, the invention is a method for fabricating high aspect ratio tapered nanocone structures including depositing at least first and second hardmasks on a substrate, the thicknesses and materials of the first and second hardmasks selected to provide etch selectivity with respect to the substrate. A nanoscale pattern of photoresist is created on the first hardmask. The first hardmask is etched through to create the nanoscale pattern on the second hardmask and the second hardmask is etched through to create the desired tapered nanocone structures in the substrate.
In a preferred embodiment of this aspect of the invention the etching is reactive ion etching. Suitable reactive ion etching is CMOS plasma etching. The substrate may be glass and either a single side or both sides of the glass may be etched depending on the particular application. Superhydrophilic or superhydrophobic, and anti-reflective, surfaces may be created.
In another aspect, the invention is a method for manufacturing glass with micro/nanostructured surface texture. Glass above its transition temperature is provided and the glass passes through a roller imprint module, the roller imprint module including a roller having a periodic structure on its surface to imprint the periodic structure to the glass surface. The periodic structure has a selected period and height.
With reference first to
The present invention is based on using multiple-step plasma etching using shrinking masks to get more flexible choices of materials and thicknesses for better control of the height and profile of nanocone structures. With reference now to
An important feature of the present invention is utilizing mask materials that are etched, but at a much slower rate as compared to the substrate. This characteristic allows the resulting profile to be tapered.
The present invention allows the aspect ratio and profile of the nanocone structures to be precisely controlled using the multi-step etching technique with shrinking masks as disclosed herein. The fabrication method disclosed herein is compatible with and can be adapted to all conventional two-dimensional lithography techniques. Since the resist pattern is transferred to the first shrinking mask, the features can be patterned with any lithographic process. Moreover, self-assembly approaches such as colloidal lithography or block copolymer can be used to pattern the multiple-shrinking masks. The tapered nanocone structures made by the process of the invention are suitable for enhancing transmission (anti-reflectivity) and wetting properties. No matter what kind of material the substrate is made of, the pattern can be transferred to the substrate with any desired aspect ratio. Since we flexibly change both material and thickness of the hardmask layers, reactive ion etching can be conducted with appropriate gases with respect to etch rates of each hardmask, and then the desired nanocone structures may be attained with multiple functionalities. Using the current technique, it is not necessary to remove any of the hardmasks because the last hardmask is used for creating tapered nanocone structure while being etched away during reactive ion etching that is directional.
The aspect ratio and shape of the nanocone structures can be optimized to achieve better wetting or optical functions with the disclosed fabrication method . By texturing subwavelength nanocones on both sides of glass through this fabrication process and modifying their surface energies, it is possible to combine high pressure robustness of superhydrophobicity and near-perfect transparency (or anti-reflection property). The nanocone surfaces made herein can show macroscopic anti-fogging for practical applications including transparent windshields and goggles that may be self-cleaning .
Another aspect of the invention is a method for manufacturing glass with a micro/nanostructured surface texture. A principle of the method disclosed herein for glass manufacture is to integrate a roller imprint module into existing flat glass manufacturing methods. Roll-to-roll nanoimprint processes have recently been proposed to mold polymer structures, but it is believed by the inventors that this disclosure is the first to integrate these processes into a continuous textured glass manufacturing system.
An important component of the roller imprint module 26 is illustrated in
Those who are skilled in the art will recognize that the principle of the invention can be applied to any continuous manufacturing process of flat glass. For example,
The method disclosed herein can also be implemented with the commonly used fusion process . In this process a container without an orifice is used to melt raw materials, and molten glass is allowed to overflow and flow down the outside walls of a container. The molten glass is then drawn downwardly and shaped by gravity. The roller imprint module 26 can then be inserted during the annealing cycle to imprint the surface micro/nanostructure texture. The method disclosed herein is highly versatile and can be embodied in many existing manufacturing processes for flat glass. In addition, the method disclosed herein can be designed as a module component such that it can be added to existing production infrastructure already in place thereby allowing scalable, cost-effective manufacturing of micro/nanoscale textured flat glass.
The method disclosed herein can be used to make glass that has multi-function properties such as anti-glare, enhanced transmission, self-cleaning, and anti-fogging. A primary application of the invention disclosed herein is the solar power industry in which solar panels made by the disclosed processes with reduced reflection and self-cleaning properties can increase panel efficiency and reduce maintenance costs. Other uses include anti-fogging effects for car windshields and eyeware.
The aspect ratio and shape of nanocone structures disclosed herein can be optimized to achieve better wetting or better optical functions . By texturing the subwavelength nanocones on both sides of glass through this fabrication process and modifying their surface energies, it is possible to combine high pressure robustness of superhydrophobicity and near-perfect transparency (or anti-reflection property). The nanocone structures can show macroscopic anti-fogging function for practical applications including transparent windshields and goggles that are self-cleaning outside. The present invention can also be used to produce a protective glass for use with digital cameras. In this case nearly 100% of the incident light with a wide angle can be collected without any loss so that pictures with better quality may be taken, even at night. Interference affects can also be eliminated.
The numbers in brackets refer to the references appended hereto all of these references are incorporated herein by reference.
It is recognized that modifications and variations of the present invention will occur to those of ordinary skill in the art and it is intended that all such modifications and variations be included within the scope of the appended claims.REFERENCES
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1. Method for fabricating tapered nanocone structures having an aspect ratio greater than five comprising:
- depositing at least first and second hardmasks on a substrate, the thicknesses and materials of the first and second hardmasks selected to provide etch selectivity with respect to the substrate, wherein the materials of the at least first and second hardmasks etch at a rate lower than an etch rate of the substrate;
- creating a nanoscale pattern of photoresist on the first hardmask;
- etching through the first hardmask to create the nanoscale pattern on the second hardmask; and
- etching through the second hardmask to create desired tapered nanoconed structures in the substrate.
2. The method of claim 1 wherein the etching through the first hardmask and the etching through the second hardmask is reactive ion etching.
3. The method a claim 1 wherein the substrate is glass.
4. The method of claim 3 wherein both sides of the glass are etched.
5. The method of claim 3 wherein superhydrophilic or superhydrophobic surfaces are created.
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Filed: Apr 13, 2012
Date of Patent: Sep 1, 2015
Patent Publication Number: 20130025322
Assignee: Massachusetts Institute of Technology (Cambridge, MA)
Inventors: Hyungryul Choi (Cambridge, MA), Chih-Hao Chang (Cambridge, MA), Kyoo Chul Park (Cambridge, MA), Gareth H McKinley (Acton, MA), George Barbastathis (Boston, MA), Jeong-gil Kim (Cambridge, MA)
Primary Examiner: Binh X Tran
Application Number: 13/446,053
International Classification: C03C 15/00 (20060101); B81C 1/00 (20060101);